Endurance events usually last at least one hour and often times multiple hours. Examples of endurance events include hiking, running, biking, swimming, and athletic events, both competitively and training. Athletic events include races, games, and other competitions.
During an endurance event, humans sweat and can easily become dehydrated. Keeping hydrated and recovering properly are vital to training in an optimal state of health. Most endurance athletes are accustomed to feeling somewhat depleted. Often times, the workouts are intended to have the athlete struggling by the finish. Travel, altered sleep patterns, dietary changes, altitude, humidity levels, medications, and personal stressors can impact daily balance tremendously. Even if life is under control, it can be tough to truly know if one has properly nutritionally recovered from a workout or athletic event and ready to begin the next one.
Training in a dehydrated state can lead to a multitude of problems from minor to life threatening. These include exhaustion, electrolyte imbalances, gastrointestinal disturbances, headaches, kidney stones, muscular injuries, altered mental status, spasms, shock, seizures, psychoses, and even death. Endurance athletes are accustomed to suffering and often times there is a fine line between safe and unstable. Also, an athlete new to a distance or event will not be familiar with “how they should feel.” Athletes often guess at whether they have fully nutritionally recovered from an endurance event.
Most of the tools and tests that have been utilized are anecdotal and largely subjective, such as:                1) Looking at the color of the urine;                    Subjectively compare day to day            Urine color charts (extremely limited, difficult to individualize)                        2) Salivary flow;        3) Physical Assessment;                    Look in the mirror for changes in physique            Weight change on scale                        4) Body Mass Scale;        5) Blood tests;                    Invasive/painful, expensive, unable to complete at home, lengthy result times            Plasma osmolality+total body water is medical gold standard                        6) Medical grade urine test strips; and                    Able to test at home, fast results, expensive            Assess 10 parameters (usually technical and confusing for the lay person)            Ketone test strip: single parameter assessing for the presence of ketones in urine,                        7) Refractrometry of Urine;                    Device to measure urine specific gravity (now automated, yet expensive).                        
There is currently no simple and reliable method for determining whether athletes have fully nutritionally recovered from an endurance event.